1. Field of the Invention
This invention relates to a scanning device which uses a stepping motor. Such a scanning device is used in, for example, an original reading apparatus, a printer, a facsimile apparatus, etc.
2. Description of the Related Art
Conventionally, an apparatus of the type which performs magnification sub-scanning by means of a stepping motor, such as a full-line-type dot printer or an image scanner, has performed the sub-scanning of one line using n steps (n: integer) of a 2-phase or 1-2 phase excitation system. Considered here will be a facsimile apparatus which is adapted to perform, when the magnification ratio is 1/1, the sub-scanning of one line at the following levels: (a) Standard level with 8 steps per line (reading/recording resolution: 3.85 lines/mm); (b) Fine level with 4 steps per line (reading/recording resolution: 7.7 lines/mm); and (c) Super-Fine level with 2 steps per line (reading/recording resolution: 15.4 lines/mm). Supposing that the magnification sub-scanning of one line is performed by this facsimile apparatus, with a magnification ratio of 5/4, the number of steps per line will be as follows:
Standard: 8.times.5/4=10 steps/line
Fine: 4.times.5/4=5 steps/line
Super Fine: 2.times.5/4=2.5 steps/line
However, a magnification sub-scanning at the rate of 2.5 steps/line, of Super Fine level, cannot be executed, so that, in that case, sub-scanning has conventionally been performed using the rate of 2 steps/line and the rate of 3 steps/line, alternately.
Thus, in the above conventional facsimile apparatus, two different rates, i.e., 2 steps/line and 3 steps/line, are mingled with each other when performing the magnification sub-scanning of one line. This arrangement leads to the following problems:
(1) The sub-scanning line density per line is not constant. That is, the sub-scanning feeding amount when the rate is 3 steps/line is 1.5 times larger than when the rate is 2 step/line.
(2) The pulse rate of the stepping motor is not constant, so that vibration and noise are caused.
Thus, if the line period is to be kept constant, the pulse rate cannot be constant since the pulse rate for the 2 steps/line scanning has to be different from that for the 3 steps/line scanning, with the result that the stepping motor is subject to vibration;
(3) The above problems might be eliminated by augmenting the number of steps per line. That, however, would make it impossible to produce apparatuses of higher speeds or lead to an excessive increase in the production cost;
(4) It is difficult to set the magnification ratio arbitrarily.
If, for example, a reduction from B4 size (257 mm) to A4 size (210 mm) is to be performed "correctly", the number of steps per line when the magnification ratio is 1/1 has to be 210 steps/line. From a practical point of view, scanning at such a rate cannot be executed.